The only time I've had yellow is when I do a liver flush, because my liver is kicking out so much bile. So, to me, yellow = bile. But, you'd also know if it's bile because it tends to irritate skin under repeated exposure.
But, there's probably a ton of other factors to be considered before I'd venture a guess in anyone else's case. Like, what other supplements are you on, what's your diet like, etc. You might take a look at your supplements and cross reference search them with liver and bile and see if anything pops up. ;)